How to communicate between iOS App Containing Extension and Extension (not Host App)

TLDR: No, but there's a hack

There's no true interprocess communication for iOS apps, with or without extensions. NSDistributedNotification still hasn't made the trip from OS X to iOS, and probably won't.

With some extension types you can open URLs via NSExtensionContext and use them to pass data to an app that handles the URL. This brings the app to the foreground, which doesn't sound like what you want.

There is a hack that might get you what you need, though.

• Instead of writing to user defaults, write to a file in your app group directory.
• Don't just write the file directly-- use NSFileCoordinator to do coordinated writes to the file.
• Implement NSFilePresenter on an object that wants to know about changes to the file, and make sure to call [NSFileCoordinator addFilePresenter:someObject]
• Implement the optional presentedItemDidChange method on your file presenter.

If you do all of this right, you can write to this file from either the app or the extension, and then have presentedItemDidChange be automatically called in the other one. As a bonus you can of course read the contents of that file, so you can pass arbitrary data back and forth.

There is a hack that you can use to communicate between any two apps in iOS or app and extension. The only thing - it doesn't work with NetworkExtension since Apple is blocking any I/O in it.

You can post notification to the DarwinNotificationCenter this way:

    let notificationName = CFNotificationName("com.notification.name" as CFString)    let notificationCenter = CFNotificationCenterGetDarwinNotifyCenter()    CFNotificationCenterPostNotification(notificationCenter, notificationName, nil, nil, false)

In your app add observer:

    let notificationName = "com.notification.name" as CFString    let notificationCenter = CFNotificationCenterGetDarwinNotifyCenter()    CFNotificationCenterAddObserver(notificationCenter,                                    nil,                                    { (                                        center: CFNotificationCenter?,                                        observer: UnsafeMutableRawPointer?,                                        name: CFNotificationName?,                                        object: UnsafeRawPointer?,                                        userInfo: CFDictionary?                                        ) in                                        print("Notification name: \(name)")                                    },                                    notificationName,                                    nil,                                    CFNotificationSuspensionBehavior.deliverImmediately)

Some links:https://github.com/choefele/CCHDarwinNotificationCenter

https://developer.apple.com/documentation/corefoundation/1542572-cfnotificationcentergetdarwinnot

https://developer.apple.com/library/content/documentation/Darwin/Conceptual/MacOSXNotifcationOv/DarwinNotificationConcepts/DarwinNotificationConcepts.html

For an alternative means of doing general-purpose bidirectional communication between host app and app extension, try MMWormhole:

http://www.mutualmobile.com/posts/mmwormholehttps://github.com/mutualmobile/MMWormhole

It’s a fairly lightweight wrapper around CFNotificationCenter, and uses “Darwin” notifications to do interprocess communication (IPC).

It passes payloads back & forth using the apps’ shared container, and encapsulates even having to create the file(s) themselves.

The class (and the sample app in the repo) seem to work well, and are quite responsive.

I hope this also helps.